Light guide optical device

ABSTRACT

The invention provides an optical device, including a light-transmitting substrate, optical means for coupling light into the substrate by total internal reflection, and a plurality of partially reflecting surfaces carried by the substrate, characterized in that the partially reflecting surfaces are parallel to each other and are not parallel to any of the edges of the substrate.

FIELD OF THE INVENTION

The present invention relates to substrate-guided optical devices, andparticularly to devices which include a plurality of reflecting surfacescarried by a common light-transmissive substrate, also referred to as alight-guide.

The invention can be implemented to advantage in a large number ofimaging applications, such as, for example, head-mounted and head-updisplays, cellular phones, compact displays, 3-D displays, compact beamexpanders as well as non-imaging applications such as flat-panelindicators, compact illuminators and scanners.

BACKGROUND OF THE INVENTION

One of the important applications for compact optical elements is inhead-mounted displays wherein an optical module serves both as animaging lens and a combiner, in which a two-dimensional display isimaged to infinity and reflected into the eye of an observer. Thedisplay can be obtained directly from either a spatial light modulator(SLM) such as a cathode ray tube (CRT), a liquid crystal display (LCD),an organic light emitting diode array (OLED), or a scanning source andsimilar devices, or indirectly, by means of a relay lens or an opticalfiber bundle. The display comprises an array of elements (pixels) imagedto infinity by a collimating lens and transmitted into the eye of theviewer by means of a reflecting or partially reflecting surface actingas a combiner for non-see-through and see-through applications,respectively. Typically, a conventional, free-space optical module isused for these purposes. Unfortunately, as the desired field-of-view(FOV) of the system increases, such a conventional optical modulebecomes larger, heavier, bulkier and therefore, even for moderateperformance device, inpractical. This is a major drawback for all kindsof displays but especially in head-mounted applications, wherein thesystem must necessarily be as light and as compact as possible.

The strive for compactness has led to several different complex opticalsolutions, all of which, on the one hand, are still not sufficientlycompact for most practical applications, and, on the other hand, suffermajor drawbacks in terms of manufacturability. Furthermore, theeye-motion-box of the optical viewing angles resulting from thesedesigns is usually very small—typically less than 8 mm. Hence, theperformance of the optical system is very sensitive, even to smallmovements of the optical system relative to the eye of the viewer, anddo not allow sufficient pupil motion for conveniently reading text fromsuch displays.

DISCLOSURE OF THE INVENTION

The present invention facilitates the design and fabrication of verycompact light-guide optical elements (LOE) for, amongst otherapplications, head-mounted displays. The invention allows relativelywide FOV's together with relatively large eye-motion-box values. Theresulting optical system offers a large, high-quality image which alsoaccommodates large movements of the eye. The optical system offered bythe present invention is particularly advantageous because it issubstantially more compact than state-of-the-art implementations and yetit can be readily incorporated, even into optical systems havingspecialized configurations.

The invention also enables the construction of improved head-up displays(HUDs). Since the inception of such displays more than three decadesago, there has been significant progress in the field. Indeed, HUDs havebecome popular and they now play an important role, not only in mostmodern combat aircraft, but also in civilian aircraft, in which HUDsystems have become a key component for low-visibility landingoperation. Furthermore, there have recently been numerous proposals anddesigns for HUDs in automotive applications where they can potentiallyassist the driver in driving and navigation duties. Nevertheless,state-of-the-art HUDs suffer several significant drawbacks. All HUD's ofthe current designs require a display source that must be offset asignificant distance from the combiner to ensure that the sourceilluminates the entire combiner surface. As a result, thecombiner-projector HUD system is necessarily bulky, and large, andrequires a considerable installation space, which makes it inconvenientfor installation and at times even unsafe to use. The large opticalaperture of conventional HUDs also pose a significant-optical designchallenge, rendering the HUD's with either a compromising performance,or leading to high cost wherever high-performance is required. Thechromatic dispersion of high-quality holographic HUD's is of particularconcern.

An important application of the present invention relates to itsimplementation in a compact HUD, which alleviates the aforementioneddrawbacks. In the HUD design of the current invention, the combiner isilluminated with a compact display source that can be attached to thesubstrate. Hence, the overall system is very compact and can be readilyinstalled in a variety of configurations for a wide range ofapplications. In addition, the chromatic dispersion of the display isnegligible and, as such, can operate with wide spectral sources,including a conventional white-light source. In addition, the presentinvention expands the image so that the active area of the combiner canbe much larger than the area that is actually illuminated by the lightsource.

Another important application of the present invention is in providingfor a large screen with a true three-dimensional (3-D) view. Ongoingdevelopments in information technology have led to an increasing demandfor 3-D displays. Indeed, a broad range of 3-D equipment is already onthe market. The available systems, however, require users to wearspecial devices to separate the images intended for left eye and theright eye. Such “aided viewing” systems have been firmly established inmany professional applications. Yet further expansion to other fieldswill require “free viewing” systems with improved viewing comfort andcloser adaptation to the mechanisms of binocular vision.State-of-the-art solutions to this problem suffer from variousdisadvantages and they fall short familiar 2-D displays in terms ofimage quality and viewing comfort. However, using the present inventionit is possible to implement a real high-quality 3-D autostereoscopicdisplay that requires no viewing aids and that can readily be fabricatedwith standard optical manufacturing processes.

A further application of the present invention is to provide a compactdisplay with a wide FOV for mobile, hand-held application such ascellular phones. In today's wireless internet-access market, sufficientbandwidth is available for full video transmission. The limiting factorremains the quality of the display within the end-user's device. Themobility requirement restricts the physical size of the displays, andthe result is a direct-display with a poor image viewing quality. Thepresent invention enables, a physically very compact display with a verylarge virtual image. This is a key feature in mobile communications, andespecially for mobile Internet access, solving one of the mainlimitations for its practical implementation. Thereby the presentinvention enables the viewing of the digital content of a full formatInternet page within a small, hand-held device, such as a cellularphone.

The broad object of the present invention, therefore, is to alleviatethe drawbacks of state-of-the-art compact optical display devices and toprovide other optical components and systems having improvedperformance, according to specific requirements.

The invention therefore provides an optical device an optical device,comprising a light-transmitting substrate having at least two majorsurfaces and edges; optical means for coupling light into said substrateby total internal reflection, and at least one partially reflectingsurface located in said substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in connection with certain preferredembodiments, with reference to the following illustrative figures sothat it may be more fully understood.

With specific reference to the figures in detail, it is stressed thatthe particulars shown are by way of example and for purposes ofillustrative discussion of the preferred embodiments of the presentinvention only, and are presented in the cause of providing what isbelieved to be the most useful and readily understood description of theprinciples and conceptual aspects of the invention. In this regard, noattempt is made to show structural details of the invention in moredetail than is necessary for a fundamental understanding of theinvention. The description taken with the drawings are to serve asdirection to those skilled in the art as to how the several forms of theinvention may be embodied in practice.

In the drawings:

FIG. 1 is a side view of a generic form of prior art folding opticaldevice;

FIG. 2 is a side view of an exemplary light-guide optical element inaccordance with the present invention;

FIGS. 3A and 3B illustrate the desired reflectance and transmittancecharacteristics of selectively reflecting surfaces used in the presentinvention for two ranges of incident angles;

FIG. 4 illustrates the reflectance curves as a function of wavelengthfor an exemplary dichroic coating;

FIG. 5 illustrates a reflectance curve as a function of incident anglefor an exemplary dichroic coating;

FIG. 6 illustrates the reflectance curves as a function of wavelengthfor another dichroic coating;

FIG. 7 illustrates a reflectance curve as a function of incident anglefor another dichroic coating;

FIG. 8 is a schematic sectional-view of a reflective surface accordingto the present invention;

FIGS. 9A and 9B are diagrams illustrating detailed sectional views of anexemplary array of selectively reflective surfaces;

FIG. 10 is a diagram illustrating a detailed sectional view of anexemplary array of selectively reflective surfaces wherein a thintransparent layer is cemented at the bottom of the light-guide opticalelement;

FIG. 11 illustrates detailed sectional views of the reflectance from anexemplary array of selectively reflective surfaces, for three differentviewing angles;

FIG. 12 is a sectional view of an exemplary device according to thepresent invention, utilizing a half-wavelength plate for rotating thepolarization of incoming light;

FIG. 13 shows two graphs representing simulated calculations for thebrightness as a function of FOV across the image of the projecteddisplay, and the external (see-through) scene;

FIG. 14 is a diagram illustrating a light-guide optical element (LOE)configuration having an array of four partially reflecting surfaces,according to the present invention;

FIG. 15 is a diagram illustrating a light-guide optical elementconfiguration having an array of four partially reflecting surfaces,according another embodiment of the present invention;

FIG. 16 is a diagram illustrating a method to expand a beam along bothaxes utilizing a double LOE configuration;

FIG. 17 is a side view of a device according to the present invention,utilizing a liquid-crystal display (LCD) light source;

FIG. 18 is a diagram illustrating an optical layout of a collimating andfolding optical element according to the present invention;

FIG. 19 is a diagram illustrating the footprint of the light, which iscoupled into the substrate, on the front surface of the collimating lensaccording to the present invention;

FIG. 20 is a diagram illustrating an equivalent, unfolded diagram of anoptical layout according to the present invention;

FIG. 21 is a diagram illustrating a diagram of an optical layoutaccording to the present invention utilizing two pairs of parallelreflecting mirrors to achieve a wide field of view;

FIG. 22A is a top view and 22B is a side view of an alternativeconfiguration for expanding light according to the present invention;

FIG. 23 illustrates an exemplary embodiment of the present inventionembedded in a standard eye-glasses frame;

FIG. 24 is a diagram illustrating an exemplary method for embedding anembodiment of the present invention within a mobile hand-held devicesuch as a cellular telephone;

FIG. 25 illustrates an exemplary HUD system in accordance with thepresent invention;

FIG. 26 illustrates an exemplary embodiment of the present inventionwhere the light-guide optical element is illuminated with an array ofdisplay sources;

FIGS. 27-29 are diagrams illustrating exemplary embodiment of an imagingsystem which projects a three-dimensional image to the eyes of a viewer,according to the present invention;

FIG. 30 illustrates an embodiment for conventional implementation of astar's-light amplifier (SLA) device;

FIG. 31 illustrates an exemplary embodiment for an improvedimplementation of star's-light amplifier (SLA) using devices accordingto the present invention;

FIG. 32 is a side view of a device according to the present invention,utilizing a reflective liquid-crystal display (LCD) display source witha conventional illuminating device;

FIG. 33 is a side view of a device according to the present invention,utilizing a reflective liquid-crystal display (LCD) display source inwhich a light-guide element is used for illuminating the source;

FIG. 35 is a diagram illustrating a method for fabricating an array ofselectively reflecting surfaces according to the present invention;

FIG. 35 is a diagram illustrating a measurement arrangement utilizingtwo prisms to measure the reflectance of a coated plate at two differentangles, and

FIG. 36 is a diagram illustrating a measurement system utilizing twoprisms to measure the reflectance of a coated plate at two differentangles further employing a folding prism to align the second output beamwith the incident input beam.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 illustrates a conventional folding optics arrangement, whereinthe substrate 2 is illuminated by a display source 4. The display iscollimated by a collimating lens 6. The light from the display source 4is coupled into substrate 2 by a first reflecting surface 8, in such away that the main ray 10 is parallel to the substrate plane. A secondreflecting surface 12 couples the light out of the substrate and intothe eye of a viewer 14. Despite the compactness of this configuration,it suffers significant drawbacks; in particular only a very limited FOVcan be affected. As shown in FIG. 1, the maximum allowed off-axis angleinside the substrate is:

$\begin{matrix}{{\alpha_{\max} = {\arctan \left( \frac{T - d_{eye}}{2l} \right)}},} & (1)\end{matrix}$

wherein T is the substrate thickness;

d_(eye) is the desired exit-pupil diameter, and

l is the distance between reflecting surfaces 8 and 12.

With angles higher than α_(max) the rays are reflected from thesubstrate surface before arriving at the reflecting surface 12. Hence,the reflecting surface 12 will be illuminated at an undesired directionand ghost images appear.

Therefore, the maximum achievable FOV with this configuration is:

FOV_(max)≈2vα_(max),  (2)

wherein v is the refractive index of the substrate.Typically the refractive index values lie in the range of 1.5-1.6.

Commonly, the diameter of the eye pupil is 2-6 mm. To accommodatemovement or misalignment of the display, a larger exit-pupil diameter isnecessary. Taking the minimum desirable value at approximately 8-10 mm,the distance between the optical axis of the eye and the side of thehead, l, is, typically, between 40 and 80 mm. Consequently, even for asmall FOV of 8°, the desired substrate thickness would be of the orderof 12 mm.

Methods have been proposed to overcome the above problem. These include,utilizing a magnifying telescope inside the substrate and non-parallelcoupling directions. Even with these solutions, however, and even ifonly one reflecting surface is considered, the system thickness remainslimited by a similar value. The FOV is limited by the diameter of theprojection of the reflective surface 12 on the substrate plane.Mathematically, the maximum achievable FOV, due to this limitation, isexpressed as:

$\begin{matrix}{{{FOV}_{\max} \approx \frac{{T\; \tan \; \alpha_{sur}} - d_{eye}}{R_{eye}}},} & (3)\end{matrix}$

wherein α_(sur) is the angle between the reflecting surface and thenormal- to the substrate plane, and

R_(eye) is the distance between the eye of the viewer and the substrate(typically, about 30-40 mm).

Practically tan α_(sur) cannot be much larger than 1; hence, for thesame parameters described above for a FOV of 8°, the required substratethickness here is on the order of 7 mm, which is an improvement on theprevious limit. Nevertheless, as the desired FOV is increased, thesubstrate thickness increases rapidly. For instance, for desired FOVs of15° and 30° the substrate limiting thickness is 18 mm or 25 mm,respectively.

To alleviate the above limitations, the present invention utilizes anarray of selectively reflecting surfaces, fabricated within alight-guide optical element (LOE). FIG. 2 illustrates a sectional viewof an LOE according to the present invention. The first reflectingsurface 16 is illuminated by a collimated display 18 emanating from alight source (not shown) located behind the device. The reflectingsurface 16 reflects the incident light from the source such that thelight is trapped inside a planar substrate 20 by total internalreflection. After several reflections from the surfaces of thesubstrate, the trapped waves reach an array of selectively reflectingsurfaces 22, which couple the light out of the substrate into the eye ofa viewer 24. Assuming that the central wave of the source is coupled outof the substrate 20 in a direction normal to the substrate surface 26,and the off-axis angle of the coupled wave inside the substrate 20 isα_(in), then the angle α_(sur2) between the reflecting surfaces and thenormal to the substrate plane is:

$\begin{matrix}{\alpha_{{sur}\; 2} = {\frac{\alpha_{i\; n}}{2}.}} & (4)\end{matrix}$

As can be seen in FIG. 2, the trapped rays arrive at the reflectingsurfaces from two distinct directions 28, 30. In this particularembodiment, the trapped rays arrive at the reflecting surface from oneof these directions 28 after an even number of reflections from thesubstrate surfaces 26, wherein the incident angle β_(ref) between thetrapped ray and the normal to the reflecting surface is:

$\begin{matrix}{\beta_{ref} = {{{90{^\circ}} - \left( {\alpha_{i\; n} - \alpha_{{sur}\; 2}} \right)} = {{90{^\circ}} - {\frac{\alpha_{i\; n}}{2}.}}}} & (5)\end{matrix}$

The trapped rays arrive at the reflecting surface from the seconddirection 30 after an odd number of reflections from the substratesurfaces 26, where the off-axis angle is α′_(in)=180°−α_(in) and theincident angle between the trapped ray and the normal to the reflectingsurface is:

$\begin{matrix}{\beta_{ref}^{\prime} = {{{90{^\circ}} - \left( {\alpha_{i\; n}^{\prime} - \alpha_{{sur}\; 2}} \right)} = {{{90{^\circ}} - \left( {{180{^\circ}} - \alpha_{i\; n} - \alpha_{{sur}\; 2}} \right)} = {{{- 90}{^\circ}} + {\frac{3\alpha_{i\; n}}{2}.}}}}} & (6)\end{matrix}$

In order to prevent undesired reflections and ghost images, it isimportant that the reflectance be negligible for one of these twodirections. The desired discrimination between the two incidentdirections can be achieved if one angle is significantly smaller thenthe other one. Two solutions to this requirement, both exploiting thereflection properties of S-polarized light were previously proposed,however, both of these solutions suffer drawbacks. The main disadvantageof the first solution is the relatively large number of reflectingsurfaces required to achieve an acceptable FOV. The main drawback of thesecond configuration is the undesired reflectance of the rays having aninternal angle of α_(in). An alternative solution is presentlydescribed, exploiting the reflection properties of P-polarized light andin some cases also the S-polarized light, and providing for a shallowerreflecting surface inclination so that fewer reflecting surfaces arerequired for a given application.

The reflection characteristics as a function of incident angle of S- andP-polarized light are different. Consider for example an air/crown glassinterface; while both polarizations reflect 4% at zero incidence, theFresnel reflectance of S-polarized light incident on the boundary risesmonotonically to reach 100% at grazing incidence, the Fresnelreflectance of P-polarized light first decreases to 0% at the Brewster'sangle and only then rises to 100% at grazing incident. Consequently, onecan design a coating with high reflectance for S-polarized light at anoblique incident angle and near-zero reflectance for a normal incidence.Furthermore one can also readily design a coating for a P-polarizedlight with very low reflectance at high incident angles and a highreflectance for low incident angles. This property can be exploited toprevent undesired reflections and ghost images as described above, byeliminating the reflectance in one of the two directions. For examplechoosing β_(ref)˜25° from Equations (5) and (6) it can be calculatedthat:

β′_(ref)=105°; α_(in)=130°; α′_(in)=50°; α_(sur2)=65°.  (7)

If now a reflecting surface is determined for which β′_(ref) is notreflected but β_(ref) is, the desired condition is achieved. FIGS. 3Aand 3B illustrate the desired reflectance behavior of selectivelyreflecting surfaces. While the ray 32 (FIG. 3A), having an off-axisangle of β_(ref)˜25°, is partially reflected and coupled out of thesubstrate 34, the ray 36 (FIG. 3B), which arrives at an off-axis angleof β′_(ref)˜75° to the reflecting surface (which is equivalent toβ′_(ref)˜105°), is transmitted through the reflecting surface 34 withoutany notable reflection.

FIG. 4 shows the reflectance curves of a dichroic coating designed toachieve the above reflectance characteristics, for four differentincident angles: 20°, 25°, 30° and 75°, all of them for P-polarizedlight. While the reflectance of the high-angle ray is negligible overthe entire relevant spectrum, the rays at off-axis angles of 20°, 25°and 30° obtain almost constant reflectance of 26%, 29% and 32%respectively, over the same spectrum. Evidently, reflectance decreaseswith the obliquity of the incident rays.

FIG. 5 illustrates the reflectance curves of the same dichroic coating,as a function of the incident angle for P-polarized light with thewavelength λ=550 nm. Evidently, there are two significant regions inthis graph: between 50° and 80° where the reflectance is very low andbetween 15° and 40° where the reflectance increases monotonically withdecreasing incident angles. Hence, as long as, for a given FOV, one canensure that the entire angular spectrum of β′_(ref), where very lowreflections are desired, will be located inside the first region whilethe entire angular spectrum of β_(ref), where higher reflections arerequired, will be located inside the second region one can ensure thereflection of only one substrate modes into the viewer's eye and ensurea ghost-free image.

Up to now, only P-polarized light was analyzed. This treatment issufficient for a system using a polarized display source, such as aliquid-crystal-display (LCD) or for a system where the output brightnessis not crucial and the S-polarized light can be filtered out. However,for an unpolarized display source, like a CRT or an OLED, and where thebrightness is critical, S-polarized light cannot be neglected and itmust be taken into account during the design procedure. Fortunately,although it is more challenging than the P-polarized light, it is alsopossible to design a coating with the same behavior for an S-polarizedlight as discussed above. That is, a coating having a very lowreflectance for an entire angular spectrum of β′_(ref) and higher,pre-defined reflections for the respective angular spectrum of β_(ref).

FIGS. 6 and 7 illustrate the reflectance curves of the same dichroiccoating described above with reference to FIGS. 4 and 5, but now forS-polarized light. Apparently, there are some differences between thebehavior of the two polarizations: the region of high angles where thereflectance is very low is much narrower for the S-polarization; it ismuch more difficult to achieve a constant reflectance for a given angleover the entire spectral bandwidth for the S-polarized light than forthe P-polarized light; and finally, the monotonic behavior of theS-polarized light at the angular spectrum of β_(ref), where higherreflections are required, is opposite to that of the P-polarized light,that is, the reflectance for the S-polarized light increases with theobliquity of the incident rays. Apparently, this contradicting behaviorof the two polarizations at the angular spectrum of β_(ref) could beutilized during the optical design of the system to achieve the desiredreflectance of the overall light according to the specific requirementsof each system.

It is clear that the reflectance of the first reflecting surface 16(FIG. 2) should be as high as possible, so as to couple as much light aspossible from the display source onto the substrate. Assuming that thecentral wave of the source is normally incident onto the substrate,i.e., α₀=180°, then the angle α_(sur1) between the first reflectingsurface and the normal to the substrate plane is:

$\begin{matrix}{{\alpha_{{sur}\; 1} = \frac{\alpha_{i\; n} + \alpha_{0}}{2}};\mspace{14mu} {\alpha_{{sur}\; 1}^{\prime} = {\frac{\alpha_{i\; n}^{\prime} + \alpha_{0}}{2}.}}} & (8)\end{matrix}$

The solutions for α_(sur1) and α′_(sur1) in the above example are 155°and 115°, respectively.

FIG. 8 presents a sectional view of the reflective surface 16 whichcouples light 38 from a display source (not shown) and traps it insidethe substrate 20 by total internal reflection. As plotted here, theprojection S₁ of the reflecting surface on the substrate surface 40 is:

S ₁ =T·tan(α),  (9)

wherein T is the substrate thickness.

The solution of α=α′_(sur1) is preferred, since the coupling area on thesubstrate surface for the above example is more than 4.5 times largerthan it is for the previous solutions. A similar improvement ratio holdsfor other systems. Assuming that the coupled wave illuminates the entirearea of the reflecting surface, after reflection from the surface 16, itilluminates an area of 2S₁=2T tan(α) on the substrate surface. On theother hand, the projection of a reflection surface 22 on the substrateplane, is S₂=T tan(α_(sur2)). To avoid either overlapping or gapsbetween the reflecting surfaces, the projection of each surface isadjacent to its neighbor. Hence, the number N of reflecting surfaces 22through which each coupled ray passes during one cycle (i.e., betweentwo reflections from the same surface of the substrate) is:

$\begin{matrix}{N = {\frac{2S_{1}}{S_{2}} = {\frac{2{T \cdot {\tan \left( \alpha_{{sur}\; 1} \right)}}}{T \cdot {\tan \left( \alpha_{{sur}\; 2} \right)}}.}}} & (10)\end{matrix}$

In this example, where α_(sur2)=65° and α_(sur1)=115°, the solution isN=2; that is, each ray passes through two different surfaces during onecycle. This is a conceptual change and a significant improvement overour previous disclosures, where each ray passes through six differentsurfaces during one cycle. The ability to reduce the number ofreflecting surfaces for a given FOV requirement relates to theprojection of the reflecting surface on the viewing plane—as the anglesin the present disclosure are larger, fewer reflection surfaces areneeded to span the image dimensions. Allowing fewer reflection surfacessimplifies the implementation of the LOE and ensures a significant costsaving in its manufacture.

The embodiment described above with regard to FIG. 8 is an example of amethod for coupling the input waves into the substrate. Input wavescould, however, also be coupled into the substrate by other opticalmeans, including (but not limited to) folding prisms, fiber opticbundles, diffraction gratings, and other solutions.

Also, in the example illustrated in FIG. 2, the input waves and theimage waves are located on the same side of the substrate. Otherconfigurations are envisioned in which the input and the image wavescould be located on opposite sides of the substrate. It is alsopossible, in certain applications, to couple the input waves into thesubstrate through one of the substrate's peripheral sides.

FIG. 9A is a detailed sectional view of an array of selectivelyreflective surfaces which couple light trapped inside the substrate outand into the eye of a viewer. As can be seen, in each cycle the coupledray passes through reflecting surfaces 42, having a direction ofα′_(in)=130°, whereby the angle between the ray and the normal to thereflecting surfaces is ˜75° and the reflections from these surfaces arenegligible. In addition, the ray passes in each cycle twice through thereflecting surface 44 having a direction of α_(in)=50°, where theincident angle is 25° and part of the ray's energy is coupled out of thesubstrate. Assuming that one array of two selectively reflectingsurfaces 22 is used to couple the light onto the viewer's eye, themaximal FOV is:

$\begin{matrix}{{FOV}_{\max} \approx {\frac{{2{T \cdot \tan}\; \alpha_{{sur}\; 1}} - d_{eye}}{R_{eye}}.}} & (11)\end{matrix}$

Hence, for the same parameters of the examples above, the limitingsubstrate thickness for an FOV of 8° is in the order of 2.8 mm; for FOVsof 15° and 30°, the limiting substrate thickness is 3.7 mm and 5.6 mm,respectively. These are more favorable values than the limitingthickness of the state-of-the-art solutions discussed above. Moreover,more than two selectively reflecting surfaces can be used. For example,for three selectively reflecting surfaces 22, the limiting substratethickness for FOVs of 15° and 30° is approximately 2.4 mm and 3.9 mm,respectively. Similarly additional reflecting surfaces may be introducesto, amongst other advantages, reduce the limiting optical thicknessfurther.

For configuration where a relatively small FOV is required, a singlepartially reflecting surface can be sufficient. For example, for asystem with the following parameters: R_(eye)=25 mm; α_(sur)=72° and T=5mm, a moderate FOV of 17° can be achieved even with a single reflectingsurface 22. Part of the rays will cross the surface 22 several timesbefore being coupled out into the desired direction. Since the minimalpropagation angle inside the substrate to achieve the total-internalreflection condition for BK7 material or similar is α_(in(min))=42°, thepropagation direction of the central angle of the FOV isα_(in(cen))=48°. Consequently, the projected image is not normal to thesurface but is rather inclined to 12° off-axis. Nevertheless, for manyapplication this is acceptable.

As illustrated in FIG. 9B, each selectively reflecting surface isilluminated by optical rays of different intensities. While the rightsurface 46 is illuminated by rays immediately after they are reflectedfrom the lower face 48 of the substrate 20, the left surface 50 isilluminated by rays that have already passed through the partiallyreflecting surface 46 and therefore have lower intensity. To achieveimages of uniform brightness, compensation is required for thedifferences in intensities between the different portions of the image.Indeed, coating the reflecting surfaces with different coatings, wherebythe reflectance of surface 46 is lower than the reflectance of surface50 compensates for the uneven illumination.

Another potential non-uniformity in the resulting image might occur dueto the different reflection sequences of different rays that reach eachselectively reflecting surface:—some rays arrive directly without areflection from a selectively reflecting surface; other rays arriveafter one or more such reflections. This effect is illustrated in FIG.9A. A ray intersects the first selectively reflecting surface 22 at thepoint 52. The incident angle of the ray is 25° and a portion of theray's energy is coupled out of the substrate. The ray then intersectsthe same selectively reflecting surface at point 42 at an incident angleof 75° without noticeable reflection, and then intersects again at point54 at an incident angle of 25° where another portion of the ray's energyis coupled out of the substrate. In contrast the ray shown in FIG. 9B,experiences only one reflection from the same surface. We note that moremultiple reflections occur at smaller incident angles. Therefore, onemethod to compensate for non-uniformity that results from suchmultiple-intersections is to design a coating where the reflectanceincreases monotonically with decreasing incident angle, as shown in thereflectivity for the range 10-40° of FIG. 5. It is difficult to fullycompensate for such differences in multiple-intersection effects.Nevertheless, in practice, the human eye tolerates significantvariations in brightness which remain unnoticed. For near-to-eyedisplays, the eye integrates all the light which emerges from a singleviewing angle and focuses it onto one point on the retina, and since theresponse curve of the eye is logarithmic, small variations, if any, inthe display's brightness will not be noticeable. Therefore, even formoderate levels of illumination uniformity within the display, a humaneye experiences a high-quality image. The required moderate uniformitycan readily be achieved with an LOE.

For displays located at a distance from the eye, however, like head-updisplays, the non-uniformity due to the multiple intersection effectscannot be tolerated. For these cases, a more systematic method toovercome the non-uniformity is required. FIG. 10 illustrate one possibleapproach. A thin transparent layer 55 with a thickness T_(add) iscemented at the bottom of the LOE. In this arrangement, the exemplaryray incident at 25°, which according to FIG. 9A intersected the firstselectively reflecting surface 22 at three points, intersects thissurface only twice and is reflected only once: at the point 52. In thismanner, the double-reflection effect does not occur. The thicknessT_(add) can be calculated to minimize the double-reflection effect forthe entire FOV of the optical system. For example, for optical systemhaving the following parameters: FOV=24°; α_(sur)=64°; α_(in)=52°;v=1.51 and T=4 mm, a layer with a thickness of T_(add)=2.1 mm should beadded to totally eliminate the undesired double-pass effect. Evidently,the total thickness of the LOE is now 6.1 mm instead of 4 mm, but forHUD systems where the combiner is relatively large and a mechanicalstrength is required for the LOE, the increased thickness is notnecessarily a drawback. It is possible to add the transparent layer atthe top of the LOE or even on both sides of the substrate, wherein theexact configuration will be set according to the specific requirementsof the optical system. For the proposed configuration, no matter whatthe thickness of T_(add) is, at least some of the rays intersect thesame selectively reflecting surface twice. For instance, in FIG. 10, theray passes once through the first reflecting surface 22 at the point 52having an incident angle of 25° where part of the ray's energy iscoupled out of the substrate, and once at an incident angle of 75°without noticeable reflection. Naturally, only the first intersectioncontributes to the image which is formed by the LOE.

In considering the viewing angles different portions of the resultingimage originate at different portions of the partially reflectingsurfaces, FIG. 11 illustrates this effect: a sectional view of a compactLOE display system based on the proposed configuration. Here a singleplane wave 56, representing a particular viewing angle 58, illuminatesonly part of the overall array of partially reflecting surfaces 22.Thus, for each point on the partially reflecting surface, a nominalviewing angle is defined, and the reflectance is designed according tothis angle.

The design of the coatings of the various partially reflective surfacesof the LOE is performed as follows: For each particular angle, a ray isplotted (taking into account the refraction due to Snell's Law) from thecenter of the designated eye pupil 60 to the partially reflectingsurface. The calculated direction is set as the nominal incidentdirection and the particular coating is designed according to thatdirection, by also taking into account prior reflectance related to thisparticular viewing angle. Hence, for each viewing angle, the averagereflectance from the relevant surfaces will be very close to the desiredreflectance. In addition, if required, a layer with a thickness T_(add)will be added to the LOE.

An LOE with non-identical selectively reflecting surfaces has twoconsequences. In see-through systems, such as head-mounted displays forpilots, wherein the viewer should see the external scene through the LOEso the reflectance of the selectively reflecting surfaces should berelatively high. Since here the reflectance coefficient is not the samefor all the selectively reflecting surfaces, there is a danger that thiswould also entail a non-uniform image of the external scene viewedthrough the substrate. Fortunately, this non-uniformity is rather small,and can be neglected in many cases. In other situations, where suchpotential non uniformity is crucial, a complementary non-uniform coatingcould be added on the external surface of the substrate, to compensatefor the non-uniformity of the substrate and to achieve a view of uniformbrightness over the entire FOV.

In non-see-through systems, such as virtual-reality displays, thesubstrate is opaque and the transmittance of the system has noimportance. However, in such a case, the reflectance may be ratherhigher than before, and care must be taken in order to ensure thatenough intensity passes through the first reflecting surface in order toachieve a uniform brightness across the entire FOV. Another issue thatshould be taken into account is the polarization of the light. Asdiscussed above, for the selectively reflecting surface coating,P-polarized light is preferred. Fortunately, some of the compact displaysources (e.g., nematic liquid crystal displays) are linearly polarized.This would also apply to situations where the display source is orientedsuch that the incoming light is S-polarized in relation to thereflective surfaces. In such cases it is possible to either design acoatings for the S-polarized light, or, alternatively rotate thesource's polarization with a half-wave plate. As illustrated in FIG. 12,the light emerging from the display source 4 is linearly S-polarized. Byusing a half-wave plate 62, the polarization is rotated so that thedesired P-polarized light is incident onto the coupling reflectivesurface 22.

To illustrate the expected performance of a typical see-through system,a computer simulation has been performed, calculating the brightness ofboth the projected display and the external scene. The system has thefollowing parameters: T=4.3 mm; T_(add)=0; α_(in)=50°; FOV=24°;R_(eye)=25 mm; v=1.51; the display source is S-polarized, there are twoselectively reflecting surfaces, and the nominal reflectance is 22%.FIG. 13 shows the simulation results, normalized to the requestednominal values. There are some small fluctuations in both graphs, butthese changes would no be noticeable in near-to-eye applications.

Thus far, only the FOV along the ξ axis has been discussed. The FOValong the orthogonal η axis should also be considered. The FOV along theη axis is not dependent upon the size or number of the selectivelyreflecting surfaces, but rather, on the lateral dimension along the ηaxis of the input waves coupled into the substrate. The maximumachievable FOV along the η axis is:

$\begin{matrix}{{{FOV}_{\max} \approx \frac{D_{\eta} - d_{eye}}{R_{eye} + {l/\left( {\nu \; \sin \; \alpha_{i\; n}} \right)}}},} & (12)\end{matrix}$

wherein D_(η) is the lateral dimension along η axis of the input wavescoupled into the substrate.

That is, if the desired FOV is 30°, then by using the same parametersdescribed above, the limiting lateral dimension is 42 mm. It waspreviously demonstrated that the longitudinal dimension along the ξ axisof the input waves coupled into the substrate is given by S₁=Ttan(α_(in)). A substrate thickness of T=4 mm yields S₁=8.6 mm.Seemingly, the lateral extent of the LOE is fivefold larger than thelongitudinal dimension. Even for an image aspect ratio of 4:3 (as with astandard video display) and the FOV in the η axis is 22°, the requiredlateral dimension is approximately 34 mm, still four-fold larger thanthe longitudinal dimension. This asymmetry is problematic:—a collimatinglens with a high numerical aperture, or a very large display source arerequired. In any case, with such numerical values dimensions, it isimpossible to achieve the desired compact system.

An alternative method for solving this problem is presented in FIG. 14.Instead of using an array of reflecting surfaces 22 only along the ξaxis, another array of reflecting surfaces 22 a, 22 b, 22 c, 22 d ispositioned along the η axis. These reflecting surfaces are locatednormal to the plane of substrate 20 along the bisector of the ξ and ηaxes. The reflectance of these surfaces is determined so as to achieveuniform output waves. For example, for four reflecting surfaces, thereflectance of the surfaces should be 75%, 33%, 50% and 100% for thefirst surface 22 a, the second surface 22 b and the third surface 22 c,and the fourth surface 22 d, respectively. This arrangement yields asequence of wavefronts, each at 25% of the incoming intensity.Typically, such an array of reflecting surfaces can be readily designedfor S-polarized light. Fortunately, the light which is S-polarizedcompared to the partially reflecting surfaces 22 a-22 d, will beP-polarized compared to the partially reflecting surfaces 22. Therefore,if the vertical expansion of the image in the η axis is affected withS-polarized light, there is no need for a half-wavelength plate torotate the polarization of the light onto the horizontal expanders inthe ξ axis. The arrangements shown in the array assemblies 22 and 22a-22 d are only examples. Other arrangements for increasing the lateraldimensions of the optical waves in both axes, in accordance with theoptical system and the desired parameters, are possible, some of whichare described below.

FIG. 15 illustrates an alternative method to expand the beam along the ηaxis. In this configuration the reflectance of surfaces 22 a, 22 b and22 c is 50% for S-polarized light where 22 d is a simple 100% mirror.Although the lateral dimension of the vertical expansion for thissolution is larger than the previous configuration, it requires only onesimple selectively reflecting coating and the overall configuration iseasier to fabricate. In general, for each specific optical system theexact method to expand the beam along the η axis can be chosen accordingto the particular requirements of the system.

Assuming a symmetrical collimating lens 6, the lateral dimension alongthe η axis after the reflection from the reflective surfaces 22 a-22 d,is given by S_(η)=NT tan(α_(in)), wherein N is the number of thereflecting surfaces. The maximum achievable FOV along the η axis is now:

$\begin{matrix}{{{FOV}_{\max}^{\eta} \approx \frac{S_{\eta} - d_{eye}}{R_{eye} + {l/\left( {\nu \; \sin \; \alpha_{i\; n}} \right)}}} = {\frac{{{NT}\; {\tan \left( \alpha_{i\; n} \right)}} - d_{eye}}{R_{eye} + {l/\left( {\nu \; \sin \; \alpha_{i\; n}} \right)}}.}} & (13)\end{matrix}$

Since the reflecting array 22 a-22 d can be located closer to the eye,it is expected that the distance l between the reflecting surfaces willbe smaller than in previous examples. Assuming that l=40 mm, andchoosing the parameters: T=4 mm; N=4; α_(in)=65°; R_(eye)=25 mm andv=1.5, the resultant FOV will be:

FOV_(max) ^(η)≈30°.  (14)

This is an improvement with respect to the previous values obtainedabove.

FIG. 16 illustrates another method to expand the beam along both axesutilizing a double LOE configuration. The input wave is coupled into thefirst LOE 20 a by the first reflecting surface 16 a and then propagatingalong the ξ axis. The partially reflecting surfaces 22 a couple thelight out of 20 a and then the light is coupled into the second LOE 20 bby the reflecting surface 16 b. The light is then propagates along the ηaxis, and then coupled out by the selectively reflecting surfaces 22 b.As shown, the original beam is expanded along both axes where theoverall expansion is determined by the ratio between the lateraldimensions of the elements 16 a and 22 b respectively. The configurationgiven in FIG. 16 is just an example of a double-LOE setup. Otherconfigurations where two or more LOEs are combined together to formcomplicated optical systems are also possible. For example, threedifferent substrates, the coating of each being designed for one of thethree basic colors, can be combined to produce a three-color displaysystem. In that case, each substrate is transparent with respect to theother two colors. Such a system can be useful for applications in whicha combination of three different monochromatic display-sources isrequired in order to create the final image. There are many otherexamples in which several substrates can be combined together to form amore complicated system.

Another issue to be addressed is the brightness of the system. Thisissue is important for see-through applications, where it is desiredthat the brightness of the display will be comparable to that of theexternal scene, to allow acceptable contrast ratio and convenientobservation through the combiner. It is not possible to ensure that theinsertion loss of most of the systems is small. For example, asdescribed above for the four-surface combiner of FIG. 14, because of therequested beam expansion along the η axis, the brightness of the opticalwaves is reduced four-fold. In general for N-reflecting surfaces thebrightness reduces by a factor of N. In principle high-brightnessdisplay sources can offset this difficulty, but this approachnecessarily has a practical limitation. Not only are high-brightnessdisplay sources very expensive, they also have high power consumptionwith the associated very high electrical currents. Furthermore, in mostof the displays there is an inherent limitation to the maximalbrightness that can be achieved. As an example, for transmission LCDs,which are presently the most abundant source for small displays, theback-illumination light power is limited to avoid undesired effects likeflaring which decrease the resolution and contrast ratio of the display.Therefore, other approaches are required to optimize the use of theavailable light from the source.

One possible method to improve the brightness of the display whichreaches the viewer's eye is to control the reflectance of the reflectingsurfaces 22 of the LOE according to the eye-motion-box (EMB) of theviewer. As illustrated in FIG. 11, each reflecting surface of theoverall array of selectively reflecting surfaces 22, is illuminated byonly the part of the overall FOV. Hence, the reflectance of each surfacecan be set to optimize the brightness of the entire FOV. For example,the reflectance of the right surface 22 a in FIG. 11 could be designedto have higher reflectance for the right part of the FOV and the lowestpossible reflectance for the left part of the FOV, while the leftsurface 22 b have higher reflectance for the left part of the FOV. Asimilar design method can be applied to a two-dimensional expansionsystem. Assuming that η is the vertical axis in FIG. 16, the reflectanceof the reflecting surfaces 22 a could be designed such that the lowersurfaces will have higher reflectance for the lower part of the FOV andthe lowest possible reflectance for the higher part of the FOV, whilethe upper surfaces have higher reflectance for the upper part of theFOV. Therefore, the factor in which the brightness is reduced because ofthe lateral expansion can be much smaller than R, where R is the ratiobetween the area of the coupling-in surface 16 a and the coupling-outsurfaces 22 b.

Another method to improve the overall brightness of the system is bycontrolling the display source brightness without changing the inputpower. As shown in FIG. 11 above, a large portion of the energy coupledonto the substrate 20 by the reflecting mirror 16 is reflected into thevicinity of the eye pupil 60. To maximize the achievable brightness,however, it is also desirable that most of the light that emerges fromthe display source couples into the substrate.

FIG. 17 illustrates an example of a substrate-mode display where thedisplay source is a transmission LCD. The light which emerges from thelight source 64 and collimated by a lens 66, illuminates an LCD 68. Theimage from the LCD is collimated and reflected by the optical components70 onto the substrate 20. FIG. 18 illustrates an optical layout of thecollimating/folding lens 70, while FIG. 19 illustrates the foot-print ofthe light, which is coupled into the substrate 20, on the front surface72 of the lens 70. Usually, for most of the display source, there is aLambertian distribution of the light, which emerges from the display.That is, the energy is distributed uniformly over the entire angularspectrum of 2π steradians. As can be seen in FIGS. 18 and 19, however,only a small portion of the light which emerges from the display sourceis actually coupled into the substrate 20. From each point source on thedisplay surface, only a small cone of light of ˜20-30° actuallyilluminate the footprint on the front surface 72 and couples into thesubstrate 20. Therefore, a significant increase in the brightness can beachieved if the light which emerges from the display is concentratedinside this cone.

One method to achieve such directionality in the source illumination isto use a special selective diffuser for the LCD. Usually, a conventionaldiffuser scatters the light uniformly in all directions. Alternatively,a selective diffuser can spread the light in such a way that the lightfrom each point source diverges into a required angular cone. In thiscase the power that the LCD surface illuminates remains the same. For a20-30° cone, the diverging angle of the light for each point source isreduced by a factor of more than 50 as compared to the π steradians ofthe Lambertian source, the brightness of the light increases by the samefactor. Hence, a significant improvement in the brightness of the systemcan be achieved with a minimal design and manufacturing effort andwithout increasing the power consumption of the system.

An alternative solution, which is appropriate not only to LCDs but alsoto other display sources, is to use an array of micro-lenses that isaligned with the pixels of the display source. For each pixel amicro-lens narrows the diverging beam that emerges from that pixel intothe desired angular cone. In fact, this solution is efficient only ifthe fill-factor of the pixels is a small number. An improved version ofthis solution is to design the emitting distribution function of thepixels in the pixel-array to make each pixel diverge into the requiredangle. For example, in OLED displays, efforts are usually made toincrease the divergence angle of the single LEDs in order to allowviewing from a wide angle. For our specific LOE display application,however, it is advantageous to keep this divergence angle small, in theorder of 20-30°, to optimize the brightness of the system.

As described above with a reference to FIGS. 14 and 15, it is possibleto achieve a wide FOV also along the vertical η direction withoutincreasing the volume of the system significantly. There are, however,situations where this solution is not sufficient. This is trueespecially for systems with a very wide FOV and a constraint on thedistance, l, between the couple-in reflective surface 16 and thecouple-out selectively reflecting surfaces 22. FIG. 20 illustrates anunfolded optical system with the following parameters: l=70 mm; T=4 mm;α_(in)=65°; R_(eye)=24 mm; v=1.51, the eye-motion-box (EMB) is 10 mm andthe required vertical FOV is 42°. If we trace the rays from the EMB 74,we find that the light passes through the projection of the EMB on thecouple-out optics 22, where 76, 78 and 80 are the projections of theupper, central and lower angles, respectively of the FOV. This meansthat to achieve the desired FOV the required couple-in aperture 82 is 65mm; this is a very large aperture that necessarily increases the size ofthe entire system, even if the substrate remains a thin plate.Alternatively, if only a smaller aperture 84 of 40 mm is allowed, theachievable vertical FOV 86 falls to 23° which is nearly half of therequired FOV.

FIG. 21 illustrates a possible solution to this problem. Instead ofusing a simple rectangular plate 20, the two horizontal edges of theplates are replaced with two pairs of parallel reflecting surfaces, 88a, 88 b and 90 a, 90 b respectively. While the central part of the FOVprojects directly through to the aperture 84 as before, the rays fromthe lower part of the FOV are reflected from surfaces 88 a and 88 b,while the rays from the upper part of the FOV are reflected fromsurfaces 90 a and 90 b. Typically, the angles between the rays trappedinside the substrate and the reflecting surfaces 88 and 90 aresufficiently large to affect total internal reflections, so no specialreflecting coating is required for these surfaces. Since all rays areeither traveling directly from the input aperture or reflected twicefrom a pair of parallel surfaces, the original direction of each ray ismaintained, and the original image is not affected.

Indeed, it is important to ensure that each ray which is reflected bysurface 88 a is also reflected by surface 88 b before it impinges onaperture 84. To confirm this, it is sufficient to check two rayspaths:—the marginal ray of the extreme angle 92, incident on surface 88a at the point 94, must impinge on surface 88 b to the right of itsintersection with surface 90 a; in addition, the marginal ray 96,incident on surface 88 a next to its intersection 98 with surface 90 b,must impinge on surface 88 b before it crosses the aperture 84. As bothmarginal rays meet the requirement, necessarily all rays from the FOVthat are incident on surface 88 a will also impinge on surface 88 b. Thepresent example provides for an FOV of 42° with a significantly reducedinput aperture 84: 40 mm. Naturally, in cases where l is extremelylarge, a cascade of two or more pairs of reflecting surfaces can be usedto achieve the desired FOV while maintaining an acceptable inputaperture.

The embodiment of FIG. 21 is just an example illustrating a simpleimplementation of this method. The use of pairs of parallel reflectingsurfaces in order to decrease the aperture of the system for a givenFOV, or alternatively to increase the useable FOV for a given aperture,is not limited to substrate-mode optics and it can be utilized in otheroptical systems including, but not limited to, free-space systems likehead-up displays, episcops or periscopes.

Apparently, as described above with reference to FIG. 21, the lateraldimension of the input aperture of the substrate is 40 mm along the ηaxis and 8.5 mm along the ξ axis. FIGS. 22A and 22B illustrate analternative embodiment to that described above with reference to FIGS.14-15. This approach involves an adjustment between a symmetricalcollimating lens 6 and an asymmetrical input aperture. The lateraldimensions of the input aperture are assumed to be D and 4D along thetwo axes respectively. A lens 6 with an aperture of 2D collimates theimage onto the substrate. The front half of the collimated light iscoupled into the substrate by the mirror 16 a. Two pairs of parallelreflecting surfaces, 22 a; 22 b and 22 c; 22 d split the coupled lightoutward and then reflects it back to its original direction. The rearpart of the collimated light passes through the substrate 20 and thenfolded by the prism 99 back into the substrate. A second mirror 16 bcouples the folded light onto the substrate 20. Evidently, the lateraldimensions of the input aperture are D and 4D along the two axesrespectively, as required.

There are some advantages to the approach describe above with referenceto FIG. 22. The system is symmetrical about the η axis and moreimportant, there is no loss of light intensity. This approach is only anexample and other similar methods to convert the symmetrical input beaminto an asymmetrical coupled light beam are possible. A suitableconfiguration for expanding the image along the η axis requires carefulanalysis of the system specifications.

In general, all the different configurations of the light-guide opticalelements considered above, offer several important advantages overalternative compact optics for display applications, which include:

1) The input display source can be located very close to the substrate,so that the overall optical system is very compact and lightweight,offering an unparalleled form-factor.2) In contrast to other compact display configurations, the presentinvention offers flexibility as to location of the input display sourcerelative to the eyepiece. This flexibility, combined with the ability tolocate the source close to the expanding substrate, alleviates the needto use an off-axis optical configuration that is common to other displaysystems. In addition, since the input aperture of the LOE is muchsmaller than the active area of the output aperture, the numericalaperture of the collimating lens 6 is much smaller than required for acomparable conventional imaging system. Consequently a significantlymore convenient optical system can be implemented and the manydifficulties associated with off-axis optics and high numerical-aperturelenses, such as field or chromatic aberrations can be compensated forrelatively easily and efficiently.3) The reflectance coefficients of the selectively reflective surfacesin the present invention are essentially identical over the entirerelevant spectrum. Hence, both monochromatic and polychromatic, lightsources may be used as display sources. The LOE has a negligiblewavelength-dependence ensuring high-quality color displays with highresolutions.4) Since each point from the input display is transformed into a planewave that is reflected into the eye of the viewer from a large part ofthe reflecting array, the tolerances on the exact location of the eyecan be significantly relaxed. As such, the viewer can see the entirefield-of-view, and the eye-motion-box can be significantly larger thanin other compact display configurations.5) Since a large part of the intensity from the display source iscoupled into the substrate, and since a large portion of this coupledenergy is “recycled” and coupled out into the eye of the viewer, adisplay of comparatively high brightness can be achieved even withdisplay sources with low power consumption.

FIG. 23 illustrates an embodiment of the present invention in which theLOE 20 is embedded in an eye-glasses frame 100. The display source 4,the collimating lens 6, and the folding lens 70 are assembled inside thearm portions 102 of the eye-glasses frame, just next to the edge of theLOE 20. For a case in which the display source is an electronic elementsuch as a small CRT, LCD, or OLED, the driving electronics 104 for thedisplay source might be assembled inside the back portion of the arm102. A power supply and data interface 106 is connectable to arm 102 bya lead 108 or other communication means including radio or opticaltransmission. Alternatively, a battery and miniature data linkelectronics can be integrated in the eye-glasses frame.

The embodiment described above can serve in both see-through andnon-see-through systems. In the latter case opaque layers are located infront of the LOE. It is not necessary to occlude the entire LOE,typically only the active area, where the display is visible needs to beblocked. As such, the device can ensure that the peripheral vision ofthe user is maintained, replicating the viewing experience of a computeror a television screen, in which such peripheral vision serves animportant cognitive function. Alternatively, a variable filter can beplaced in front of the system in such a way that the viewer can controlthe level of brightness of the light emerging from the external scene.This variable filter could be either a mechanically controlled devicesuch as a folding filter, or two rotating polarizers, an electronicallycontrolled device, or even an automatic device, whereby thetransmittance of the filter is determined by the brightness of theexternal background.

There are some alternatives as to the precise way in which an LOE can beutilized in this embodiment. The simplest option is to use a singleelement for one eye. Another option is to use an element and a displaysource for each eye, but with the same image. Alternatively it ispossible to project two different parts of the same image, with someoverlap between the two eyes, enabling a wider FOV. Yet anotherpossibility is to project two different scenes, one to each eye, inorder to create a stereoscopic image. With this alternative, attractiveimplementations are possible, including 3-dimensional movies, advancedvirtual reality, training systems and others.

The embodiment of FIG. 23 is just an example illustrating the simpleimplementation of the present invention. Since the substrate-guidedoptical element, constituting the core of the system, is very compactand lightweight, it could be installed in a vast variety ofarrangements. Hence, many other embodiments are also possible includinga visor, a folding display, a monocle, and many more. This embodiment isdesignated for applications where the display should be near-to-eye:head-mounted, head-worn or head-carried. There are, however,applications where the display is located differently. An example ofsuch an application is a hand-held device for mobile application, suchas for example a cellular phone. These devices are expected in the nearfuture to perform novel operations, which require the resolution of alarge screen, including videophone, Internet connection, access toelectronic mail, and even the transmission of high-quality televisionsatellite broadcasting. With the existing technologies, a small displaycould be embedded inside the phone, however, at present, such a displaycan project either video data of poor quality only, or a few lines ofInternet or e-mail data directly into the eye.

FIG. 24 illustrates an alternative method, based on the presentinvention, which eliminate the current compromise between the small sizeof mobile devices and the desire to view digital content on a fullformat display, by projecting high quality images directly into the eyeof the user. An optical module including the display source 6, thefolding and collimating optics 70 and the substrate 20 is integratedinto the body of a cellular phone 110, where the substrate 20 replacesthe existing protective cover-window of the phone. Specifically, thevolume of the support components including source 6 and optics 70 issufficiently small to fit inside the acceptable volume for moderncellular devices. To view the full screen transmitted by the device theuser positions the window in front of his eye 24, to conveniently viewthe image with high FOV, a large eye-motion-box and a comfortableeye-relief. It is also possible to view the entire FOV at a largereye-relief by tilting the device to display different portions of theimage. Furthermore, since the optical module can operate in see-throughconfiguration, a dual operation of the device is possible; namely it isoptionally possible to maintain the conventional cellular display 112intact. In this manner the standard, low-resolution display can beviewed through the LOE when the display source 6 is shut-off. In asecond mode, designated for e-mail reading. Internet surfing, or videooperation, the conventional display 112 is shut-off while the displaysource 6 projects the required wide FOV image into the viewer's eyethrough the LOE. The embodiment described in FIG. 24 is only an example,illustrating that applications other than head-mounted displays can bematerialized. Other possible hand-carried arrangements include palmcomputers, small displays embedded into wristwatches, a pocket-carrieddisplay having the size and weight reminiscent of a credit card, andmany more.

The embodiments described above are mono-ocular optical systems, thatis, the image is projected onto a single eye. There are, however,applications, such as head-up displays (HUD), wherein it is desired toproject an image onto both eyes. Until recently, HUD systems have beenused mainly in advanced combat and civilian aircraft. There have beennumerous proposals and designs, of late, to install a HUD in front of acar driver in order to assist in driving navigation or to project athermal image into his eyes during low-visibility conditions. Currentaerospace HUD systems are very expensive, the price of a single unitbeing in the order of hundreds of thousands of dollars. In addition, theexisting systems are very large, heavy, and bulky, and are toocumbersome for installation in a small aircraft let alone a car.LOE-based HUD potentially provide the possibilities for a very compact,self-contained HUD, that can be readily installed in confined spaces. Italso simplifies the construction and manufacturing of the opticalsystems related to the HUD and therefore is a potentially suitable forboth improving on aerospace HUD's, as well as introducing a compact,inexpensive, consumer version for the automotive industry.

FIG. 25 illustrates a method of materializing an HUD system based on thepresent invention. The light from a display source 4 is collimated by alens 6 to infinity and coupled by the first reflecting surface 16 intosubstrate 20. After reflection at a second reflecting array (not shown),the optical waves impinge on a third reflecting surfaces 22, whichcouples the light out into the eyes 24 of the viewer. The overall systemcan be very compact and lightweight, of the size of a large postcardhaving a thickness of a few millimeters. The display source, having avolume of a few cubic centimeters, can be attached to one of the cornersof the substrate, where an electric wire can transmit the power and datato the system. It is expected that the installation of the presented HUDsystem will not be more complicated than the installation of a simplecommercial audio system. Moreover, since there is no need for anexternal display source for image projection, the necessity to installcomponents in unsafe places is avoided.

Since the exit pupil of a typical HUD system is much larger than that ofa head-mounted system, it is expected that a three-array configuration,as described above with reference to FIGS. 14-16, will be needed toachieve the desired FOV There may be some special cases, however,including systems with small vertical FOVs, or with a vertical LED arrayas a display source, or by exploiting pairs of parallel reflectingmirrors (as described above with reference to FIG. 21) in which atwo-array configuration would suffice.

The embodiments illustrated in FIG. 25 can be implemented for otherapplications, in addition to HUD systems for vehicles. One possibleutilization of these embodiments is as a flat display for a computer ortelevision. The main unique characteristic of such a display is that theimage is not located at the screen plane, but is focused at infinity orto a similarly convenient distance. One of the main drawbacks ofexisting computer displays is that the user has to focus his eyes at avery close distance of between 40 and 60 cm, while the natural focus ofa healthy eye is to infinity. Many people suffer from headaches afterworking for a long duration of time at a computer. Many others who workfrequently with computers tend to develop myopia. In addition, somepeople, who suffer from both myopia and hyperopia, need specialspectacles for work with a computer. A flat display, based on thepresent invention, could be an appropriate solution for people whosuffer from the above-described problems and do not wish to work with ahead-mounted display. Furthermore, the present invention allows for asignificant reduction in the physical size of the screen. As the imageformed by the LOE is larger than the device, it would be possible toimplement large screens on smaller frames. This is particularlyimportant for mobile applications such as lap and palm-top computers.

One potential problem that might arise with a large display LOB relatesto its brightness. Ideally, for compactness it is advantageous to use aminiature display source, but this necessarily reduces the displaybrightness due to the large increase in the actively illuminated area ofthe LOE as compared to the actively illuminated area of the source.Therefore, even after the special measures described in the foregoingare deployed, one expects a reduction in the brightness, even fornon-see through applications. This reduction in the brightness can beoffset either by increasing the brightness of the source, or deployingmore than one source. That is, the LOE can be illuminated with an arrayof display sources and their associated collimating lenses. FIG. 26illustrates an example of this method. The same image is generated froman array of 4 display sources 4 a through 4 d, each collimated by arelated array of lenses 6 a through 6 d to form a single collimatedimage, which is coupled into the LOE 20 by the reflecting surface 16. Ata first glance it looks like this solution can be quite expensive. Hereany increased system cost through increase in its components and theneed to coordinate the sources images with special electronics is offsetby the inherently low cost of the micro-displays themselves and theability to reduce the numerical aperture of the collimating lenses.There is also no need for a lateral expander in this arrangement; it isquite feasible to include only a one-dimensional image expander LOE andincrease the brightness accordingly. It is important to note that thedisplay sources should not necessarily be identical to each other and amore complicated system with a different display sources can be utilizedas explained in what follows.

Another advantage of the LOE display of the present invention is itsvery flat shape, even compared to the existing flat-panel displays.Another difference is a significantly more directional viewing angle:the LOE display can be viewed from a significantly limited angular rangeas compared to common flat-panel display. Such limited head-motion-boxis sufficient for convenient operation by a single user, and offers theadditional advantages of privacy in many situations.

Furthermore, the image of the LOE-based screen is located in a distantplane behind the display surface and not on its physical surface. Thesensation of the image is similar to viewing it through a window. Thisconfiguration is particularly suitable for implementingthree-dimensional displays.

Ongoing developments in information technology have led to an increasingdemand for 3-D displays. Indeed, a broad range of 3-D equipment isalready on the market. The available systems, however, suffer from thedrawback that users are required to wear special devices to separateimages intended for the left eyes and the right eye. Such “aidedviewing” systems have been firmly established in many professionalapplications. Yet further expansion to other fields will require “freeviewing” systems with improved viewing comfort and closer adaptation tothe mechanisms of binocular vision. The present solutions to thisproblem suffer from various disadvantages and fall behind familiar 2-Ddisplays in terms of image quality and viewing comfort.

FIGS. 27A and 27B illustrate a front view and a top view, respectively,of a possible configuration, based on the present invention tomaterialize a real 3-D display. Instead of a single display source, anarray 114 of n different display sources 114 ₁ 114 _(n) is located atthe lower portion of the substrate 20, where each display sourceprojects images obtained at different perspectives of the same scene.The image from each display source is coupled into the substrate in thesame manner as described above with reference to FIG. 26. When theviewer is observing the display, his right 24 a and left 24 b eyes viewthe images projected from the display sources 114 i and 114 j,respectively. Consequently the viewer sees with each eye the same scenefrom a different perspective. The experience closely resembles theviewing experience when observing a real 3-D object through a window. Asillustrated in FIGS. 28 a-28 b, when the viewer moves his gazehorizontally his eyes see the images which are projected from differentdisplay sources 114 k and 114 l; the effect is similar to moving thehead across a window while looking at an external scene. When the viewermoves his gaze vertically, as illustrated in FIGS. 29A-29B, the eyes seepoints on the screen which are located lower then before. Since thesepoints are located closer to the display sources 114, the viewer seesimages which emerge from different display sources 114 g and 114 h,which are located closer to the center of the array 114 than before. Asa result, the sensation of the viewer is similar to viewing a scene,which is closer to the window. That is, the scene through the substrateis seen as a three-dimensional panorama where the lower part of thescene is closer to the viewer.

The embodiment described above with regard to FIGS. 27-29 is only anexample. Other arrangements for realizing a real 3-D display, withdifferent apertures, number of aspect points and more are also possibleby utilizing the present invention.

Another possible embodiment of the invention is its implementation as ateleprompter, such as used as to project text to a speaker or TVbroadcaster; as the teleprompter is transparent, the audience feel thatthe speaker is making eye-contact with them while he is actually readingtext. Utilizing an LOE, the teleprompter can be implemented with a smallsource, attached to the optical assembly, alleviating the need to locatelarge screen in the vicinity of the device.

Yet another possible implementation of this embodiment is as a screenfor a personal digital assistance (PDA). The size of the existingconventional screens which are presently used, is under 10 cm. Since theminimal distance where these displays can be read is on the order of 40cm, the obtainable FOV is under 15°; hence, the information content,especially as far as text is concerned, on these displays is limited. Asignificant improvement in the projected FOV can be made with theembodiment illustrated in FIG. 24. The image is focused at infinity, andthe screen can be located much closer to the eyes of the viewer. Inaddition, since each eye sees a different part of the totalfiled-of-view (TFOV), with an overlap at its center, another increase inthe TFOV may be achieved. Therefore, a display with an FOV of 40° orlarger is feasible.

In all of the embodiments of the invention described above, the imagewhich was transmitted by the substrate 20, originated from an electronicdisplay source such as a CRT or LCD. There are, however, applicationswhere the transmitted image can be a part of a living scene, forexample, when it is required to couple a living scene onto an opticalsystem.

FIG. 30 illustrates an application of star-light amplifier (SLA) 116where this implementation is required. The image from the external sceneis focused by the collimator 118 into the SLA where the electronicsignal of the image is amplified to create a synthetic image which isprojected through an eye-piece 120 onto the viewer eye. The illustratedconfiguration is fairly popular for military, para-military and civilianapplications. This commonly used configuration necessarily protrudesforward in front of the user and makes it inconvenient for protracteduse in a head-mounted configuration. The device is relatively heavy andin addition to its physically interference with objects in the vicinityof the user, and it exerts a strenuous moment on the user's head andneck.

A more convenient configuration is illustrated in FIG. 31. Here, thedevice is not located in front of the user but to the side of the head,where the center of the gravity of the SLA is aligned along themain-axis of the head. The direction of the device is reversed, that is,the collimator 118 is located at the rear and the eye-piece 120 islocated at the front. Now, the image from the frontal external scene iscoupled into the collimator 118 by using an LOE 20 a, where the imagefrom the eye-piece 120 is coupled into the user's eye by using anotherLOE 20 b. Though additional two optical elements, 20 a and 20 b, areadded to the original device, the weight of these elements is negligiblecompared to the weight of the SLA and the overall configuration is muchmore convenient than before. Furthermore, as the mounting tolerance ofthese devices is far from demanding, it is feasible that these twoelements be configures as modular so that they can be either shiftedaway from their position or even removed by the user. In this manner theSLA viewer can be reconfigured for convenient location for head-mountedoperation with the LOE mounted, or for mounting on standard gun-sites orother aiming devices for use without the LOE modules. It is alsopossible to shift the LOE's so as to accommodate the use of the devicewith either eye.

In all of the embodiments described above, the LOE is utilized totransmit light waves for imaging purposes. The present invention,however, can be applied not only for imaging, but also for non-imagingapplications, mainly illumination systems, in which the optical qualityof the output wave is not crucial and the important parameters areintensity and uniform brightness. The invention may be applied, forexample, in back illumination of flat-panel displays, mostly LCDsystems, in which, in order to construct an image, it is necessary toilluminate the plate with a light as bright and uniform as possible.Other such possible applications include, but are not limited to, flatand non-expensive substitutes for room illumination or for floodlights,illuminators for fingerprint scanners, and readout waves for3-dimensional display holograms.

One of the illumination utilizations that can be considerably improvedby using an LOE device is for a reflective LCD. FIG. 32 illustrates anexample of a substrate-mode display where the display source is areflection LCD. The light generated by an illuminator 122 passes througha polarizer 124, collimated by a lens 126, reflected by a polarizingbeamsplitter 128 and illuminates an LCD 130. The polarization of thelight which is reflected from the LCD is rotated in 90° by a ¼wavelengths plate, or alternatively by the LCD material itself. Theimage from the LCD now passes through the beamsplitter to be collimatedand reflected by the lens 132 onto the substrate 20. As a result of thebeamsplitter configuration, the entire illuminating system is large andcumbersome, and certainly not compact enough for head-mounted systems.Moreover, because of the beamsplitter 128 the collimating lens 132 islocated further away from the display source, while for the sake ofminimizing the aberrations it is required that the field-lens will belocated as close as possible to the display surface.

An improved version of the illuminating setup is illustrated in FIG. 33.The light from the light source 122 coupled into another LOE 134, whichilluminates the surface of the LCD 130, where the partially reflectivesurfaces are polarizing sensitive. Apparently, the entire system here ismuch more compact than that illustrated in FIG. 32, and the lens 132 islocated much closer to the LCD surface. In addition, since the inputaperture of the LOE 134 is much smaller than that of the beamsplitter128, the collimating lens 126 can be now much smaller than before, andtherefore have a larger f-number. The illuminating arrangement shown inFIG. 32 is only an example. Other arrangements for illuminating areflective or transmission LCD, or for using for any other illuminatingpurposes in accordance with the optical system and the desiredparameters, are also permissible.

An important issue that should be addressed is the fabricating processof the LOE, where the crucial component is the array of selectivelyreflecting surfaces 22. FIG. 34 illustrates a possible method offabricating an array of partially reflecting surfaces. The surfaces of aplurality of transparent flat plates 138 are coated with the requiredcoatings 140 and then the plates are cemented together so as to create astack form 142. A segment 144 is then sliced off the stack form bycutting, grinding and polishing, to create the desired array ofreflecting surfaces 146, which can be assembled with other elements tomaterialize the entire LOE. More than one array 146 can be fabricate ofeach segment 144, according to the actual size of the coated plates 138and the required size of the LOE. As described in FIGS. 4-7, therequired coatings of the selectively reflecting surfaces should have aspecific angular and spectral response in order to assure a properoperation of the LOE. Hence, it is essential to accurately measure theactual performance of the coatings before the final fabrication of theLOE. As explained above, there are two angular regions that should bemeasured—the high incident angles (usually between 60° and 85°) wherethe reflectance is very low and the low incident angles (usually between15° and 40°), where the reflectance of the surfaces is utilized tocouple part of the trapped waves out of the LOE. Naturally, the coatingshould be measured at these two regions. The main problem of the testingprocedure is that it is very difficult to measure with the existingtesting equipment the reflectance (or alternatively the transmission)for very high angles of incidence, usually above 60°, for coatings thatare located, as in our case, between two transparent plates.

FIG. 35 illustrates a method proposed to measure the reflection of acoated surface 150 at very high incident angles. Initially two prisms152 with an angle α are attached to the coated plate. The incoming beam154 impinges on the coated plate at an incident angle α. Part of thebeam 156 continues at the original direction and its intensity T_(α) canbe measured. Hence, taking into account the Fresnel reflections from theexternal surface, the reflectance of the measured coating at the angle αcan be calculated as R_(α)=1−T_(α). In addition, the other part of thebeam is reflected from the coated surface, reflected again by totalinternal reflection from the external surface of the lower prism,impinges at the coated surface again at an angle 3α, reflected againfrom the external surface of the upper prism by total internalreflection, and then reflected by the coated surface at an angle α andcoupled out from the prism. Here, the intensity of the output beam 158can be measured. Taking into account the Fresnel reflections, theintensity of the output beam is (R_(α))²*T_(3α). Hence, knowing thereflectance R_(α) from the previous step, the reflectance at an angle 3αcan be calculated accordingly. There are testing equipments where theoutput beam must be located at the same axis of the incoming beam. FIG.36 illustrates a folding prism 160 used to translate the beam into thatof the original beam. The residue of the original ray 154 can be blockedusing a suitable mask or blocking layer 162.

Evidently, each pair of prisms can measure the reflectance at twoangles—α and 3α. For instance, if the head angle is 25° then thereflectance at 25° and 75° can be measured simultaneously. Therefore, asmall number of prism pairs (2 or 3) is usually requested for a propermeasurements of the coated plates. Naturally, the setup shown here canbe utilized to measure the reflectance of these two angles at differentwavelengths as well as for the two polarizations, if required.

It will be evident to those skilled in the art that the invention is notlimited to the details of the foregoing illustrated embodiments and thatthe present invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

1.-52. (canceled)
 53. An optical device, comprising: a display sourceemitting light waves; a selective diffuser for non-uniformly diffusingthe light of the display source; an imaging optical module; alight-transmitting substrate having a light-admitting surface and atleast one surface located inside said substrate for coupling the lightout of said substrate; an optical coupler for coupling light waves fromsaid imaging module into said substrate, and an output aperture foremitting light from the optical device, characterized in that lightwaves from the entire display source are coupled by the optical couplervia said light-admitting surface into the substrate by total internalreflection.
 54. The optical device according to claim 53 furthercomprising at least one light source.
 55. The optical device accordingto claim 54 further comprising at least one converging lens.
 56. Theoptical device according to claim 55, wherein said converging lenscollects light from said light source and illuminates said displaysource.
 57. The optical device according to claim 56, wherein said lightdiffuser diffuses the light interposed between said converging lens andsaid display source.
 58. The optical device according to claim 56,wherein each light wave emerging from a single point on the displaysource is diverged into a predetermined finite solid angle, such thatlight waves from the entire display source are coupled into saidsubstrate by total internal reflection.
 59. The optical device accordingto claim 58, wherein optical nature and structure of the combination ofsaid light source, converging lens, display source and light diffuser,dictate said solid angles, resulting in coupling said light waves intosaid substrate by total internal reflection.
 60. The optical deviceaccording to claim 58, wherein optical nature and structure of thecombination of the components of said optical device results in couplingsaid light waves into a single eye of an observer.
 61. The opticaldevice according to claim 58, wherein optical nature and structure ofthe combination of the components of said optical device results incoupling said light waves into both eyes of an observer.
 62. The opticaldevice according to claim 60, wherein optical nature and structure ofthe combination of the components of said optical device results in aspecific brightness at the eye of the observer.
 63. The optical deviceaccording to claim 61, wherein optical nature and structure of thecombination of the components of said optical device results in aspecific brightness at the eyes of the observer.
 64. The optical deviceaccording to claim 53, wherein from each point source on said displaysurface, only a specific angular cone of light actually couples intosaid substrate.
 65. The optical device according to claim 53, whereinsaid selective diffuser spreads the light from each point source of saiddisplay source into said specific angular cone.
 66. The optical deviceaccording to claim 53, wherein input waves and image waves are locatedon the same side of said substrate.
 67. The optical device according toclaim 53, wherein input waves and image waves are located on theopposite sides of said substrate.
 68. The optical device according toclaim 53, wherein said optical device is utilized as head mounteddisplay.
 69. The optical device according to claim 53, wherein saidoptical device is utilized as head up display.